The invention relates generally to improved fluid valve assemblies, and more particularly to such valve assemblies for gas compressors. The invention also relates to a capacity modulation apparatus for such compressors. The invention is applicable, however, to such compressor valve assemblies, both with or without such a capacity modulation system.
Reciprocating piston-type gas compressors typically include pressure-actuated suction and discharge valving mounted generally at the end of the cylinder between the head assembly and the compression chamber. It is critical to the overall operation of a gas compressor to provide a sufficiently large port area to permit the flow of a maximum volume of gas within a given time period, at an acceptably small pressure drop. This is particularly true for refrigeration compressors employed in air conditioning systems because of the relatively high mass flow rates generally required in such systems. In addition to maximizing the port area for a given cylinder size, it is advantageous to reduce the weight of a moving valve member and thereby limit the inertia effect thereof, as well as to minimize the operating noise of the valve assembly, especially in high speed compressors.
In such gas compressors, it is also important to normally minimize the reexpansion or clearance volume at the valve end of the cylinder. Accordingly, the valving and the compression chamber end wall should have a shape that is complimentary with that of the top of the piston, to enable the piston to reduce the volume of the compression chamber to a minimum during the compression stroke without restricting gas flow. While it may be possible to accomplish this objective by designing a complex piston head shape, manufacturing of such complex pistons is very expensive, assembly is difficult and throttling losses frequently occur as the piston approaches top dead center. Such reduction of reexpansion volume is also of great importance in refrigeration compressors having relatively low mass flow rates, such as those employed in very low temperature refrigeration systems, as well as in heat pump applications.
Because of the criteria discussed above, the present invention seeks to provide an improved valve and valve seat combination for use as a discharge valve in gas compressors, without regard to whether or not means are provided for capacity modulation. The invention thus seeks to improve the efficiency of the gas compressor, the discharge flow characteristics at all valve lifts, the sealing characteristics of the valve without permanent deformation of the valve member, the durability of the valve assembly, the quietness and smoothness of operation, and the valve assembly's ability to operate in high speed compressors.
In many refrigeration (and heat pump) systems some type of technique is required to control or modulate the capacity of gas compressors. The need for compressor capacity modulation arises from the dilemma of a constant displacement pump being coupled to a system with varying heating and cooling demands wherein the refrigeration system functions to balance the load of heat influx at a selected temperature at the evaporator. This load will vary from minimum to maximum levels for which the system was designed.
Normally, load balance in gas compressors is accomplished with a thermal expansion valve which regulates the flow of liquid phase refrigerant in to the evaporator. If the heat load in a refrigeration systems decreases, the expansion valve reduces the refrigerant flow rate. Conversely, the expansion valve increases refrigerant flow when the heat load increases. Thus, if the system has no means for capacity control, a reduction in refrigerant flow rate will result in a lowering of suction pressure. The compressor capacity is therefore reduced as it operates at a higher compression ratio and a lower volumetric efficiency. To some extent, this reduction can be tolerated and is usually taken into consideration in the system design. However, there is a minimum suction temperature or pressure below which either the compressor or the system should not be operated, which may be the saturated suction temperature at which frost begins to form on an air-cooling evaporator coil, or perhaps the minimum temperature limitations of a water chiller.
In some cases, particularly in low temperature refrigeration systems, a compressor may overheat as the compression ratio becomes excessive with reduced flow rates. Smaller refrigeration systems can use a simplistic approach, employing a thermostat or suction pressure operated switch to start and stop the compressor to overcome these problems. In larger refrigeration systems, however, this approach is not desirable because of the resultant wide temperature fluctuations and the adverse effects on compression reliability. Therefore, in such large refrigeration systems, a sensor is used to selectively activate an unloader mechanism for modulating the capacity of the compressor without shutting it down.
Several different types of unloaders have been used in the past, including hot gas bypass unloaders, blocked suction unloaders, suction valve lifter mechanisms, and reexpansion clearance pockets. The present invention is particularly applicable to this latter type of unloader in which clearance pockets are used to provide an additional clearance or reexpansion volume, with the clearance pocket being separated from the cylinder by a stop valve or the like, in order to thereby reduce the volumetric efficiency of the compressor.
Previous designs of capacity modulation systems using clearance pockets have several drawbacks. Frequently, the hardware is bulky and impractical to use with modern, high speed compressors where compactness is an important design objective. Alternatively some previous designs rely upon manual adjustments to valves or pistons to define the additional clearance volume. While these prior art approaches may have been manageable for a relatively few slow speed compressors, the manual adjustment approach is not practical in modern day air conditioning or refrigeration systems where automatic control is desired. In addition, such previous designs have not lent themselves to be incorporated into compressors utilizing the "Discus" valving construction developed by Copeland Corporation, the assignee of the present invention. This valving arrangement is characterized by its improved flow characteristics at all valve lifts, good sealing without permanent deformation of the valve, long life, quietness in operation, and its ability to operate in high speed compressors. More details of this valve assembly can be found in U.S. Pat. No. 4,368,755 issued to King on Jan. 18, 1983, which patent is assigned to Copeland Corporation, the assignee of the present invention, and is hereby incorporated herein by reference.
Pursuant to the present invention as it applies to a capacity modulation system, a valve member for a gas compressor is formed with an opening therethrough, and a head assembly connected to the valve plate includes a clearance pocket therein. A plunger element slidably mounted in the clearance pocket is provided with a base or stem extending into an open cylindrical neck member disposed in the valve member opening. The plunger is adapted to move to a first position which closes off fluid communication between the compression chamber and the clearance pocket and thereby places the compressor in a fully loaded condition. In a second position, the plunger retracts away from the opening in the valve member to allow fluid communication between the compression chamber and the clearance pocket in order to unload the compressor.
Among the advantages of this invention is that a compact design is achieved by making the discharge valving assembly an integral part of the capacity modulation mechanism for defining the extent of the clearance volume. The mass of the discharge valve is reduced thereby reducing impact on the valve seat when the valve is closed. In the preferred embodiment, a retainer member is provided with an apertured neck that serves as a guide for the valve during valve movement and during seating on the valve seat. In addition, a sealing means is provided between the valve member and the neck member in order to sealingly and slidably dispose the valve member on the neck and to provide a dampening effect to reduce valve impact.
In one particular embodiment, a solenoid is used to quickly move the plunger to its loaded or unloaded position. Thus, capacity control is achieved automatically and without the necessity for manual adjustments. In another embodiment of the invention, inserts or sleeves of varying dimensions can be replaceably inserted into the clearance pocket to selectively determine the extent of plunger retraction, thereby defining the volume of the clearance pocket to permit various degrees of desired unloading.
The present invention is further directed toward the provision of an inexpensive and simple, high-efficiency valve apparatus for a modulated gas compressor, or other fluid flow device, wherein the advantageous features of the valve assembly described in the above-mentioned U.S. Pat. No. 4,368,755 can be fully realized. Therefore, the present invention further seeks to improve the operation of the compressor discharge valve assembly, and in particular to reduce the friction between the valve member and the neck member discussed above, thereby increasing its speed and smoothness of operation, while providing good sealing characteristics between the valve member and the neck member, even in high-temperature conditions that frequently occur.
According to the present invention in its broader aspects (i.g., without regard to capacity modulation), an improved valve assembly includes a valve member that is longitudinally movable between closed and open positions, with the valve member having an inner opening extending longitudinally therethrough for receiving a fixed neck member in order to allow guided slidable movement of the valve member on the neck member between an open valve position and a closed valve position. The valve member also includes a shoulder having a sealing surface thereon, with the sealing surface being positioned laterally outboard of the inner opening.
The above-mentioned neck member also includes a shoulder portion having a sealing surface, with a resilient sealing member sealingly engaging both the sealing surface of the valve member and the sealing surface of the neck member when the valve member is in the closed position in order to prevent leakage of gas between the valve member and the fixed neck member. The sealing member is resiliently deflectable during the slidable movement of the valve member between its open and closed positions, and the sealing member does not slidably engage the neck member. The sealing member is biased into such sealing engagement by a gas pressure differential existing across the valve member when in its closed position, as well as by a resilient biasing spring compressed between the sealing member and a fixed portion of the valve assembly.
In one preferred embodiment of the invention, the above-described valve assembly is employed as a discharge valve assembly for a reciprocating piston gas compressor, although it is fully applicable to non-reciprocating type compressors. In such embodiment, the valve assembly includes a valve plate having a valve seat opening extending therethrough with the valve member sealingly engaging the valve seat opening when in its closed position. Preferably the valve member and the neck member have generally flat longitudinally-facing surfaces on opposite sides from their respective shoulder portions, with such generally flat surfaces being substantially coplanar both with one another and with the inner surface of the valve plate when the valve member is in its closed position. These coplanar surfaces in part define a portion of the compression chamber and minimize undesirable reexpansion volume in the compression chamber when capacity modulation is either not provided or not being used.